1. Field of the Invention
The present invention relates to cellular communications systems and, more particularly, to a vestigial identification system and method for more efficiently transmitting information in cellular communications systems employing channel reuse.
2. Description of Related Art
A cellular communications system is a mobile wireless telephone service in which radio coverage is divided into a network of cells, and each cell is assigned a number of available radio frequencies. In a standard cellular communications system, a base station transceiver system transmits and receives control and voice communication data from a base station controller within the same cell, and transfers the data to selected mobile users within the cell via a radio interface, such as the E-TDMA.TM. radio interface of Hughes Network Systems, Inc.
The base station controller in such a system is controlled by a mobile switching center which provides an interface to a public switched telephone network interface, and which provides mobility control across the cellular system. The mobile switching center controls the transfer of information to selected base station controllers within the cellular system according to the position of the mobile user to whom the information is directed, providing a roaming and hand-off function between cells.
In cellular communications systems, encoding/decoding procedures are applied to transmitted voice and other data to detect data errors. If errors should go undetected, message transmission may be disrupted. One cellular system error detection procedure involves the encoding of a message using a cyclic redundancy code (CRC).
According to the CRC error detection process, information bits representing information to be transmitted are encoded by a CRC encoder to produce CRC code words. As shown in FIG. 7, each code word 70 consists of an `I` field 72 containing L.sub.i information bits, representing the data to be transmitted, and a `P` field 74 comprising L.sub.p parity check bits. For each possible combination of information bits L.sub.i, a parity check containing p parity bits is generated. Each code word 70 is formed by combining the information bits 72 and associated parity bits 74. The collection of all such code words is called the code book. To enable error detection at the receiving end, the `P` field 74, containing the CRC parity bits L.sub.p, is transmitted with the `I` field 72 information bits L.sub.i.
The entire code word 70, including the information field 72 and the parity check field 74, is transmitted through the noisy channel to a receiver. The receiver then produces an information field I' and a parity check field P', approximating the transmitted code word 70. The information field I' is subsequently CRC encoded to generate a parity check P". If P" is identical to P', the message is accepted by the receiver. This implies that the information field I' and the parity check field P' form a code word that can be found in the code book.
The performance of a CRC can be examined with respect to its distance properties. For an (n, k) CRC, "n" refers to the total length (L.sub.i +L.sub.p) of the code word, and "k" refers to the length of the information portion L.sub.i. Thus, the code book contains 2.sup.Li, or 2.sup.k, entries, and the comparison of P" to P' is equivalent to looking in the code book for an entry corresponding to I'+P'. The CRC is further characterized by a minimum distance D.sub.min, representing the minimum distance between any two code words in the code book. Specifically, the minimum distance refers to the number of bits which differ between any two code words. For instance, a minimum distance D.sub.min =7 means that there must be at least seven errors in the received message (I'+P') for the CRC to fail to detect an error. This is true because the received message (I'+P') must form another code word in the code book to be accepted.
In a cellular communication system employing channel reuse, in which data is transmitted to different cellular users via the same channels, the ability of a receiver to distinguish between a message from the appropriate transmitter and a message from an interfering transmitter using the same channel is important. Acceptance of control messages from interfering cells can cause calls to be dropped. To enable the receiver to identify the correct message in a system using channel reuse, an identifier indicating the source of the interfering transmission is generally transmitted with the message.
In some systems in which message identification and error detection are provided, an identifier field 32 and a parity check field 36 have been included with data 34 as part of the transmitted message, as shown in FIG. 3. The parity check 36 is used to detect errors in both the data field 34 and identifier field 32 of the message, but the identifier itself is also transmitted to enable the receiver to determine the origin of the received transmission. If either the identifier is incorrect or the parity check flags a bit error in the data or the identifier, the message is disregarded.
For cellular networks having high traffic densities, it would be desirable to increase the data bearing capacity of each transmission by eliminating the transmission of the identifier while continuing to provide detection of interfering messages. For example, it is not necessary that the receiver identify which interfering transmitter sent the message. Because it is only necessary that the receiver determine whether the message was transmitted from an interferer, the transmission of identifier data indicating the source of the interfering transmission may not be required.